Release of secretin in man will be studied to determine the pH threshold and to study the relationship between acid load in the duodenum and the amount of secretin released. The study will be carried out in abnormal states such as patients with achlorhydria and exocrine pancreatic insufficiency. Interaction between secretin in a physiological dose range and octapeptide of cholecystokinin on pancreatic secretion in man and dog will be investigated. Studies to document secretin as an enterogastrone will be further carried out in dog, man and rat. A factor or factors that release secretin, other than acid, will be searched for and the release mechanism of secretin will be studied in dog.